Steam fresh spinach for 5 minutes (or thaw frozen spinach completely). Drain spinach thoroughly, squeezing out as much liquid as possible. Add nutmeg and lemon juice to the cooked spinach and set aside.
In a large pot add chopped carrots, celery, onion, white wine, water, and bouquet garni. Bring liquid to a boil, lower the heat and simmer for 15 minutes. Add fish to the top of the simmering liquid and poach for no more than 5 minutes or until the fish becomes opaque.
Remove the cod from the poaching liquid. Strain and discard the vegetables. Reserve 2 cups of the strained poaching liquid and return to the pot and bring to a simmer.
Whisk the cornstarch into approximately 3 to 4 teaspoons of cold water to make a slurry. Add to the poaching liquid. Bring the poaching liquid and cornstarch mixture to a boil. Stir the mixture until desired thickness.
Add the cheddar to the thickened mixture. Season with salt and pepper, to taste.
Peel and cube potatoes. Place in a small saucepan and cover with salted water. Boil approximately 15 minutes, or until fork tender. Drain, reserving a bit of the liquid with the potatoes, and mash potatoes to smooth, but not pasty consistency. Add roasted garlic, salt and pepper, to taste.
Oil a 13 by 9-inch ovenproof casserole dish. Place the spinach on the bottom of the casserole dish. Place the cod over the spinach. Spread the cheese sauce evenly over the fish layer. Top with all of the warm mashed potatoes that will comfortably fit in the dish. Add more grated white cheddar, as desired.
Bake uncovered in a preheated 400 degrees F oven for approximately 40 minutes or until the cheese and mashed potatoes are golden brown. Remove from oven, and allow to stand for approximately 10 to 15 minutes before serving.
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A viewer or guest of the show, who may not be a professional cook, provided this recipe. It has not been tested for home use.
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